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SubjectRe: [BUG] x86 kenel won't boot under Virtual PC


On Fri, 5 Sep 2008, David Sanders wrote:
>
> The patch disables a optimization used in one place by commenting out some
> lines in nop.h. Please comment.

Hmm.. I'm not a huge fan of the ASM_NOP mess, but you also disable it for
64-bit x86 too.

On 32-bit, at least the generic nops are fairly reasonable, but the
default nops for 64-bit really look pretty sad, and the P6 nops really do
look better.

So I would suggest perhaps moving the static P6 nop selection into the
CONFIG_X86_64 thing.

The alternative is to just get rid of that static nop selection, and just
have two cases: 32-bit and 64-bit, and just pick obviously safe cases for
them.

So I think that particular part would be better off with changing the
Kconfig language instead. Ie something like this..

(I removed the 32-bit CPU's from the choices, except for MPENTIUM4 that
really should be merged with MPSC - the difference between MPENTIUM4 and
MPSC seems to be just a totally bogus 32-bit vs 64-bit thing. Yes, yes,
there are PENTIUM4's without 64-bit, but there are also Prescott chips
that run 32-bit kernels, so the thing is a bit confused.

Linus

---
arch/x86/Kconfig.cpu | 13 ++++++++-----
1 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/arch/x86/Kconfig.cpu b/arch/x86/Kconfig.cpu
index 2c518fb..b225219 100644
--- a/arch/x86/Kconfig.cpu
+++ b/arch/x86/Kconfig.cpu
@@ -382,14 +382,17 @@ config X86_OOSTORE
# P6_NOPs are a relatively minor optimization that require a family >=
# 6 processor, except that it is broken on certain VIA chips.
# Furthermore, AMD chips prefer a totally different sequence of NOPs
-# (which work on all CPUs). As a result, disallow these if we're
-# compiling X86_GENERIC but not X86_64 (these NOPs do work on all
-# x86-64 capable chips); the list of processors in the right-hand clause
-# are the cores that benefit from this optimization.
+# (which work on all CPUs). In addition, it looks like Virtual PC
+# does not understand them.
+#
+# As a result, disallow these if we're not compiling for X86_64 (these
+# NOPs do work on all x86-64 capable chips); the list of processors in
+# the right-hand clause are the cores that benefit from this optimization.
#
config X86_P6_NOP
def_bool y
- depends on (X86_64 || !X86_GENERIC) && (M686 || MPENTIUMII || MPENTIUMIII || MPENTIUMM || MCORE2 || MPENTIUM4 || MPSC)
+ depends on X86_64
+ depends on (MCORE2 || MPENTIUM4 || MPSC)

config X86_TSC
def_bool y

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